1. Field of the Invention
The present invention relates to a transformer for use in communications devices or various power supplies, and, more particularly, to a transformer which can be reduced in size and which can be made thinner by an improved shape of a magnetic core.
2. Description of the Related Art
FIGS. 8A and 8B are sectional front and sectional plan views of a conventional transformer such as those disclosed in, for example, Japanese Unexamined Utility Model Application Publication Nos. 6-70223 and 6-55222. In these figures, a coil 3 is accommodated inside of a cylindrical outer leg 1 and disposed around a middle leg 2. Transformers which are combinations of E-type magnetic cores or which are combinations of E-type magnetic cores and I-type magnetic cores, with a coil being wound upon a middle leg thereof, have also been used. In the structures of such transformers, the sizes of the transformers are obtained by adding the sizes of the outer legs of the corresponding cores to the external sizes of the corresponding coils.
However, in the conventional structure whose size is obtained by adding the size of the outer leg of the corresponding core to the external size of the corresponding coil, the footprint area and overall size of the transformer become considerably greater than those of the corresponding coil, thereby resulting in the inconvenience that the transformer cannot be made sufficiently small and thin.
To address this problem, the present invention provides a transformer which can be made small and thin as a result of making the dead space that exists when mounting a coil to a magnetic core as small as possible.
In order to overcome the above-described problems, the present invention provides the following structural features.
(1) According to a basic form of the present invention, there is provided a transformer comprising a first magnetic core and a second magnetic core. The first magnetic core includes a middle leg provided in a standing manner at the central portion of a plate, preferably flat and polygonal in shape, and outer legs provided in a standing manner at a plurality of corners of the plate, in which a portion disposed between the middle leg and the outer legs is a winding accommodating portion having a shape for accommodating a winding. The second magnetic core is placed on the first magnetic core in order to be attached thereto, for example by means of an adhesive. In the transformer, a plurality of substantially planar coils, which are each formed by winding a wire in a planar or flat shape, are inserted into the winding accommodating portion and stacked upon each other.
A cross-sectional shape of the middle leg is at least partially curved, preferably circular or substantially elliptical. A curved portion of a cross-sectional outer peripheral shape of the middle leg has the shape of a circular arc formed concentrically with a curved portion of an outer peripheral shape of the winding accommodating portion. An outer periphery of the winding accommodating portion is formed so as to be close to or substantially coincide with corresponding side portions of an external boundary of the magnetic core.
In the basic form, the transformer is formed by placing the first and the second magnetic cores upon each other, and accommodating the coil therebetween. In the first magnetic core, a middle leg and outer legs are provided in a standing manner at the central portion and at the plurality of corners of the polygonal flat plate, with a portion disposed between the middle leg and the outer legs serving as a winding accommodating portion for accommodating a winding. In the case where the transformer is constructed in this way, when the coil is formed by winding a wire in a flat manner, and is accommodated inside the winding accommodating portion, the comers thereof are located at the corresponding outer legs, as a result of which the dead space becomes small, so that the footprint area of the entire transformer becomes small.
By forming the middle leg with either a circular shape or a substantially elliptical shape, the central portion of the coil can be fitted thereto without any gap, thereby making it possible to further reduce the amount of dead space.
Similarly, by forming the curved portion of the cross-sectional outer peripheral form of the middle leg into the shape of a circular arc formed concentrically with a curved portion of the outer peripheral boundary of the winding accommodating portion, and forming the outer periphery of the winding accommodating portion so that it is close to or coincident with a side portion of the external outline of the magnetic core, the footprint of the transformer can be minimized.
Since a coil lead can be drawn out from an opening at a portion where the outer periphery of the winding accommodating portion is adjacent to a corresponding side of the first magnetic core, it is no longer necessary to provide a separate opening for passing the lead therethrough.
A plurality of leads of the planar coil may be drawn out from different openings in the first magnetic core that are not located on a same straight line. Therefore, the transformer can be disposed at a greater variety of locations.
(2) The second magnetic core may have the shape of a flat plate. In this form, the coil formed by winding a wire in a flat shape is accommodated inside the winding accommodating portion formed inside the first magnetic core. By forming the entire winding accommodating portion inside the first magnetic core, the second magnetic core can be formed with the shape of a flat plate.
Alternatively, the first and the second magnetic cores can be formed with the same shape so that a portion of the winding accommodating portion is defined in each of the first and second magnetic cores. When this is done, it is possible to manufacture one fewer component part.
A winding width of the winding accommodating portion may be greater than a thickness of the winding accommodating portion, and the width of the transformer may be greater than its height. This contributes to reducing the thickness of the transformer.
(3) An area determined by the product of a cross-sectional peripheral length of the middle leg and a thickness of the polygonal flat plate may be substantially equal to or greater than a cross-sectional area of the middle leg; or the total sum of cross-sectional areas of the outer legs may be substantially equal to or greater than the cross-sectional area of the middle leg; or the total sum of areas of inwardly facing portions of areas determined by products of cross-sectional peripheral lengths of the corresponding outer legs and the thickness of the polygonal flat plate may be substantially equal to or greater than the cross-sectional area of the middle leg.
By virtue of this structure, it is possible to restrict the reduction of induction caused by the concentration of magnetic flux at other portions of the magnetic core.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.